In a communication apparatus such as a cell phone, a piezoelectric thin-film filter using a piezoelectric thin-film resonator is beginning to be used as a bandpass filter for selectively transmitting or receiving only a desired signal. For example, an FBAR (Thin Film Bulk Acoustic Resonator) filter is known as the piezoelectric thin-film filter. In this filter, a piezoelectric thin film sandwiched between electrodes is formed on a substrate such as an Si substrate, and a gap is formed on the substrate side of the electrode formation portion in order to prevent a leak of vibration energy to the Si substrate. An SMR (Solidity Mounted Resonator) filter using an acoustic reflection film instead of a gap is also used. These filters are generically called BAW (Bulk Acoustic Wave) filters.
The operation principle of the BAW filter will briefly be explained below. When a high-frequency signal is applied between two electrodes sandwiching a piezoelectric thin film, the piezoelectric thin film resonates at a frequency corresponding to the piezoelectric thin film, thereby functioning as a resonator. When a ladder circuit is formed by connecting an interconnection to each electrode of this piezoelectric thin-film resonator, a desired filter characteristic can be obtained.
Since the BAW filter is a filter using a piezoelectric thin-film resonator, it is necessary to sufficiently increase the Q value as a quality coefficient of the piezoelectric thin-film resonator. To improve the Q value of the resonator, it is ideal to completely confine vibration energy generated by resonance in the piezoelectric thin film. In an actual piezoelectric thin-film resonator, however, the vibration energy leaks due to a propagation mode in a lateral direction perpendicular to the thickness direction, and generates unnecessary vibration (spurious).
To solve this problem, patent literature 1 suppresses the spurious caused by the lateral propagation mode by increasing the thickness of one electrode (apex electrode) in the vicinity of the edge. Also, patent literature 2 suppresses the spurious caused by the lateral propagation mode by decreasing the thickness of the piezoelectric thin film in an excitation region, and increasing the thickness in a non-excitation region in the periphery. Furthermore, patent literature 2 has a structure in which a cave is formed in a support substrate in the excitation region where the piezoelectric thin film is thinned, so vibration excited in the excitation region does not interfere with the support substrate.
In addition, in the technique of patent literature 2, a single-crystal substrate having a thickness with which the substrate can withstand its own weight is adhered to the support substrate having the gap, and thinned to a desired thickness by mechanical polishing or the like, thereby obtaining a predetermined frequency. In the technique of patent literature 2, therefore, a piezoelectric thin film is formed without film formation such as deposition, and this eliminates a problem such as the influence of the underlayer on the crystallinity. Consequently, it can be expected to obtain a good piezoelectric performance.